The detection of bacterial activity within a container as evidenced by an increase of pressure within the container is well known, particularly in the food industry wherein an increase of pressure in a food container often indicates that the level of bacterial activity is so great as to render the food unsafe. Several devices for detecting such a pressure change have been developed using mechanical, electrical and other sensing means. Some of these employ devices which must penetrate the container while others respond to movement of a wall of the container using optical or other sensing.
It is highly desirable to be able to detect activity within a vessel in a laboratory context for determining the presence, absence, or the approximate population of microorganisms. Devices for this general purpose have also been devised. It is important, first, to be sure that the interior of the vessel, once supplied with a sample of the substance to be tested, is isolated from the ambient atmosphere to prevent contamination of the substance and also to prevent contamination of laboratory workers by the substance. Thus, the known pressurechange detecting techniques which involve penetration of the testing vessel for measurement purposes are hard to use and impractical. Of the other known techniques, while they are undoubtedly suitable for the purposes for which they were developed, they are relatively insensitive to small differences of pressure between the inside and outside of the container. Thus, such systems lack adequate sensitivity and, in some cases, an inordinate amount of time would be required to incubate the substance being tested to the point at which a pressure change could be sensed.